Method and apparatus for sensing inter-modulation to improve radio performance in single and dual tuner

ABSTRACT

A method of performing alternate frequency switching in a radio includes tuning the radio to a primary frequency. A candidate alternate frequency is identified. It is determined whether the candidate alternate frequency is a third order inter-modulation artifact. Tuning is switched from the primary frequency to the candidate alternate frequency only if it is determined in the determining step that the candidate alternate frequency is not a third order inter-modulation artifact.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.14/507,983, filed on Oct. 7, 2014 which is currently under allowance,which is a continuation of U.S. patent application Ser. No. 13/892,792,filed on May 13, 2013, now U.S. Pat. No. 8,886,142, issued on Nov. 11,2014, which is continuation of U.S. patent application Ser. No.13/010,225, filed on Jan. 20, 2011, now U.S. Pat. No. 8,463,216, issuedon Jun. 11, 2013, the disclosures of which are hereby incorporated byreference in their entireties for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to radios for use in vehicles, and, moreparticularly, to improving signal reception quality in radios for use invehicles.

2. Description of the Related Art

Car radio reception quality is an important element of overall consumervehicle satisfaction. Consequently, car original equipment manufacturers(OEMs) and suppliers perform extensive in-field testing in differentcountries to tweak the reception quality to suit each market segment.

Users listening to radios while driving near an AM or FM radiotransmission tower may hear two types of distortion. A first type ofdistortion is front end overload distortion where transmission from anearby station overwhelms the car radio's RF digital signal processing(DSP) receiver. Front end overload can lead to clipping distortion atthe intermediate frequency analog-to-digital conversion chain process. Amethod to avoid this is to increase the attenuation at the front end andtweak the automatic gain control (AGC) that is prior to theanalog-to-digital conversion (ADC) stage. However, since the overloadaffects the entire FM frequency range, the radio frequency (RF) designeris presented with the challenge of accommodating both strong and weaksignal reception in such a scenario.

A second type of distortion is inter-modulation distortion arising fromthe use of non-linear devices. In the car radio environment, thenon-linear devices are primarily the low noise amplifier (LNA) and theheterodyne mixer or mixers depending on whether the heterodyne mixingprocess is one step 10.7 MHz intermediate frequency or a lower frequencydown-shifted base band intermediate frequency operating for low powerdevices using multi-stage down conversion. The current trend withrespect to low power devices is to operate in base band intermediatefrequency to ensure that the sampling rate is lower, as this translatesinto lower power utilization at the analog-to-digital conversion stageonwards.

Inter-modulation occurs when the input to a non-linear device (NLD) iscomposed of two or more frequencies of high signal levels and results inthe creation of frequency artifacts that are a product of the inputs.These artifacts can result in either new ‘phantom’ stations (e.g.,artifacts occur on frequencies where no valid station exists in thevicinity of the car radio) or overlap on a existing valid frequency.When a user tunes to the overlapped frequency, he hears audiomodulations from the multiple audio sources including the valid radiostation and the modulations arising from station frequencies involved inthe creation of the artifacts themselves.

For example, third order inter-modulation can arise from the followingpermutations:

L*f1+/−M*f2+/−N*f3, where f1, f2 and f3 are distinct frequencies andL+M+N=3, where L, M and N are integers   (1)

Here f1, f2 and f3 are signals over 70 dBuV (which may be calibratable)OR from

L*f1+/−M*f2, where f1, f2 are distinct frequencies and L+M=3 where L andM are integers   (2)

Here f1 and f2 are signals over 70 dBuV (which may be calibratable)

While inter-modulation is typically caused by a car's proximity tostrong transmission towers, other causes may originate from inside thecar's passenger compartment through the use of powerful in-car FMtransmitters which are used to stream audio from an external device(e.g., an iPod or external mp3 player) into a non-receivable FM radiostation frequency so that the external audio source can be heard throughthe car speakers. These devices may output signal levels from 70 to 90dBuV. Signals of a level exceeding 70 dBuV are considered strong signalsand when mixed with other strong signals in the vicinity of the car, canlead to third order inter-modulation artifacts.

While inter-modulation distortion in the car radio can be of secondorder and third order types, the third order inter-modulation poses abigger problem than second order inter-modulation. This is becausesecond order inter-modulation can be typically filtered out using theband-pass filter. However, third order inter-modulation is harder tofilter out as it lies very close to the center frequency of thefrequency tuned by the radio head unit. A filter with characteristicssteep enough to filter out third order inter-modulation but leave thetuned frequency intact is difficult to achieve.

Illustrated FIG. 1 is an example of typical prior art RF receivertopology that results in the creation of inter-modulation artifacts. TheRF signal from the antenna goes through a low noise amplifier (LNA),which is a non-linear device, and then goes through a band-pass filterwhich tends to filter out frequencies outside the FM band. The nextstage is the mixing with the local oscillator to provide theintermediate frequency. The mixer is also a non-linear device. Theoutput product from the mixer passes through another filter stage toensure that only the required intermediate frequency is output beforethe signal is digitally sampled at the RF analog to digital converter(ADC) and then again passes through an intermediate frequency (IF)filter.

FIG. 2 illustrates the characterization or mapping of the input powerversus output power of a typical non-linear device. The plotted line 10represents the third order inter-modulation characterization. The gainof the output inter-modulation product is based on the slope of line 10.For Global A boards, for example, the third order inter-modulation isbetween 10 and 15 dBuV and is known to cause audio distortion.

Illustrated in FIG. 3 is an example expanded characterization of outputpower versus input power for a non-linear device. FIG. 3 illustrates atypical model that is used to characterize the level of artifactscreated. Line 12 represents the third order inter-modulationcharacterization.

The level of expected inter-modulation is shown in FIG. 4, whichillustrates modeling of third order inter-modulation. The third orderinput intercept point (IIP3) is in units dBm and is a function of APfrom the input levels of the fundamental strong frequencies at the inputto the non-linear device.

FIG. 5 illustrates the third order intercept point (IP3)inter-modulation power increase for non-linear devices with nosaturation. As shown in FIG. 5, the effects of inter-modulation varybased on the RF design and the characteristics of the components used.If the system has no saturation, then the third order inter-modulationcan be as high as the fundamental frequencies at the input of thenon-linear device.

FIG. 6 also illustrates IP3 inter-modulation power increase fornon-linear devices with no saturation. As shown in FIG. 6, the thirdorder inter-modulation effects depend on the performance of the gainstages at the latter part of the RF chain. This is true because the gainvalue increases geometrically towards (G_(n)) at the end of the chain.

FIG. 7 illustrates a characterization of the problem posed by thirdorder inter-modulation. FIG. 7 highlights the reason why it is difficultto filter out the third order inter-modulation artifacts. While thesecond order harmonics are outside the pass band, the third orderinter-modulations such as 2f1−f2 and 2f2−f1 are very close to thefundamental frequencies f1 and f2 (where f1 and f2 are strong signals of70 dBuV or above). Because of the difficulty in filtering the thirdorder inter-modulations, this poses a serious reception problem.

Accordingly, what is neither anticipated nor obvious in view of theprior art is a method of sensing inter-modulation distortion andmitigating its effects on signal reception quality.

SUMMARY OF THE INVENTION

The present invention may provide a method of using known car radiohardware architecture in conjunction with a novel software algorithm tothereby sense and mitigate inter-modulation artifacts and improve theoverall performance of the car radio reception quality.

There are several end applications contemplated for the presentinvention with respect to the car radio. A first end application is toimprove single and dual tuner alternative frequency switch behavior.Global A radios have a test route which exhibits a classical use case:On Mount Taunus in Germany, there exists two strong transmittersoperating at 102.5 MHz and 105.9 MHz. These two strong signals (over 90dBuV) result in a third order inter-modulation product(2×102.5)−105.9=99.1 MHz. Also in the vicinity (2×96.7)−94.3=99.1 MHz,another intermodulation is produced on the same frequency. When the useris tuned to station SWR1 and drives up the mountain, an unwantedalternative frequency (AF) switch occurs to the strongest station (99.1MHz) which has good quality and yields a proper Program ID code prior tothe switch. However when the radio switches to this station, the userhears distorted audio artifacts where there are audio products fromthree separate stations (SWR1+station operating 102.5 MHz and stationoperating 105.9 MHz). In the above scenario, with regards to audioquality, the reception quality can be improved if the radio switches toan alternate frequency that is of secondary signal quality rather thanthe strongest quality, and that is not an inter-modulation product, thusyielding better audio quality performance.

A second end application of the invention is radio data system (RDS)preset recall/digital audio broadcasting (DAB) FM link performanceenhancement. Preset recall or DAB FM link to an RDS station involvestuning by Program ID code rather than frequency. Herein the radio checksall the best alternative frequencies associated with the Program ID codeand tunes to the best alternative frequency with the criteria beingsignal quality and the frequency transmits the Program ID code. Withinter-modulation at play the radio risks tuning to a station that is aninter-modulation product. This results in the end user tuning to astation whose audio quality is composed of the inter-modulatingfrequencies and the actual audio content.

A third end application of the invention is autoseek performanceenhancement. A car radio parked or being driven near a transmissiontower may need to ensure that it does not seek stops on inter-modulationtainted frequencies even if the quality of these stations are consideredgood and within limits with respect to field-strength levels, multipath,ultrasonic and frequency offset metrics.

A fourth end application of the invention is to optimize distortionartifacts during manual tune operation. In the event that the userespecially wants to listen to a station frequency through direct tune ormanual tune operation, the radio, upon detecting that the frequency hasinter-modulation artifacts, can choose to adjust the automatic gaincontrol to improve audio quality.

The present invention may provide a mechanism to detect inter-modulationin single tuner and dual tuner radios and utilize this aprioriinformation in avoiding the inter-modulation artifacts. The inventivemethod may accommodate the case in which the car moves away from thestrong signal transmitters, or when the in-car FM transmitters have beenturned off. The invention may enable the car radio to recognize thatinter-modulation artifacts are no longer present and thus adapt itself.

The inventive method may detect the inter-modulation and use thisapriori information to improve the performance of a number ofapplications. Specifically, the method may improve RDS AF switchingbehavior in single and dual tuner radios by ensuring that the radio doesnot switch to a tainted inter-modulation frequency. The method may alsoimprove RDS Preset recall performance by ensuring that the tune by PIcode ensures that the alternative frequency picked for reception is nota frequency tainted by inter-modulation artifacts. The method mayfurther improve auto-seek seek stop performance in the FM mode to ensurethat seek stop does not occur at a frequency associated with aninter-modulation artifact.

In Europe, DAB FM link occurs when a user is tuned to a digital DABstation. When the bit error rate (BER) increases, the decoding of theMP2 compressed audio stream becomes difficult for the DAB receiver. Insuch a circumstance, the radio typically falls back on the simulcast FMstation frequency to produce audio. FM stations in Europe employ RDSwhich categories stations with a program ID code whereby multiplefrequencies are associated with a single station. In such a case, a tuneby PI operation of the present invention to trigger the DAB FM link mayensure that the final strongest alternative frequency picked for tuneoperation in the FM band is not an inter-modulation artifact.

The present invention may be applied to AF switching in either a singleor double tuner environment. European countries embrace the fullfeatures set out by the RDS standard which is AF switching. The way thisscheme works is that low power transmitters encompass the European FMlandscape. A station operates under different frequencies whereby audioon all these alternate frequencies consists of simulcast audio and datainformation from the station.

A single tuner radio operating in this environment, when tuned to a RDSstation, may receive the AF that the radio can switch to in case thecurrently tuned-to frequency fades in signal quality. Before an actualswitch is done, the single tuner RDS radio may typically perform qualitychecks, such as for fieldstrength, multipath, adjacent channel energy,and frequency offset, for example. After the quality checks have beenperformed, and the AF is noted to be better than the currently tuned-tostation frequency, the radio may switch over to this stronger AF after amute operation and delve on this target station for a program ID codecheck. The program ID confirms that the station being switched to istransmitting the same audio as the most recently tuned-to station. Thismay result in mutes which can vary in time duration based on the timeused for the PI wait time. The mute time duration may range between 500ms and 1500 ms depending on the RDS block error rate, which may beaffected by frequency offset errors, multi-path and/or adjacent channelactivity, assuming the sampled signal is of good quality (e.g., 32 dBuVor above for field strength). If the PI code (a sixteen bit word termed“program identification code” and defined in the RBDS standard) matchesthe PI code of the last tuned-to station, then the AF switch occur, andan unmute of audio is performed. If the PI code does not match thesampled AF, then the radio switches back to the originally tuned-tostation and unmutes. The latter is a partly failed AF switch attempt asthe radio transmitter list of alternate frequencies is not fully correctbecause either these station frequencies are operating as regionalvariants, or a true case of co-channel situation exists such that thefrequencies can carry different audio content.

When the PI code cannot be received, then the alternate frequency switchmay be delayed.

Muted PI checks may be performed for single tuner variants. OEMcustomers require this program ID check partly to reduce the risk ofpotentially switching over to a different station (with different audiomodulation) and are willing to tolerate the mute. However, in order toprevent too many mutes from occurring, what is referred to as a “trusttimer” is used to perform an un-muted alternate frequency switch. Thetrust timer may minimize the number of audible mutes.

The way this scheme works is that after acquiring the PI code through amute, typically a trust timer is set for the frequency. The trust timeris usually a counting up timer starting from 0 seconds (the time atwhich the PI code is received) to a maximum of 15 minutes. The way thistrust timer helps in reducing the number of mutes is such that once asingle tuner radio sets the trust timer, the radio can potentiallyswitch over to this station frequency in what is termed an unmuted PIcheck (frequency is switched without muting the audio) during the validduration of the trust timer as specified by the developer. The durationof the trust timer specified by the developer can vary based on thelocality and proximity of the radio stations. This approach of using atrust timer may not work well, however, in certain FM landscapes whereco-channel frequency exists, e.g., where a second station uses the samealternate frequency known to the radio. In this instance, an unmuted AFswitch can result in what is termed a “wrong audio modulation” lastingfrom the time the AF switch occurs, the radio variant tunes to this newstation, senses through the reception that the new station has the wrongPI code, and finally reacts by switching back to the original frequency.To prevent the software from using the sampled frequency, what may bereferred to as a “disable timer” may be set.

In summary of the above limitations on the operation of a single tunerRDS radio, there may be mutes during a PI code switch. An unmuted AFswitch based on the trust timer can reduce mutes but does not combatagainst wrong modulation in case frequencies are reused by differentstations. Stations in Europe also operate as regional variant stations.Single tuner radio variants have these operational limitations becausethere is no luxury of a second tuner to perform background scanning andinaudible PI checks.

The method of the present invention adds a third degree of optimizationby performing the PI check only for alternative frequencies that arepresented to the car radio and that are not third order inter-modulationartifacts. The way such alternative frequencies may be sensed in asingle tuner is through the use of information gathered in the frequencylearn memory of the frequencies in the FM band.

The invention comprises, in one form thereof, a method of performingalternate frequency switching in a radio, including tuning the radio toa primary frequency. A candidate alternate frequency is identified. Itis determined whether the candidate alternate frequency is a third orderinter-modulation artifact. Tuning is switched from the primary frequencyto the candidate alternate frequency only if it is determined in thedetermining step that the candidate alternate frequency is not a thirdorder inter-modulation artifact.

The invention comprises, in another form thereof, a method of performingautoseek in a radio, including scanning a radio frequency band for acandidate frequency having a quality exceeding a threshold qualitylevel. It is determined whether the candidate frequency is a third orderinter-modulation artifact. The radio is tuned to the candidate frequencyonly if it is determined in the determining step that the candidatefrequency is not a third order inter-modulation artifact.

The invention comprises, in yet another form thereof, a method ofautomatically tuning an FM radio to a frequency, including identifying aplurality of first frequencies within an FM band that have a signalquality above a threshold level. A plurality of second frequencies thatare third order inter-modulation artifacts of the first frequencies arecalculated. Tuning to the second frequencies is avoided.

An advantage of the present invention is that it prevents the radio fromtuning to a third order inter-modulation artifact in autoseek, AFswitching, and DAB FM Link operations.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features and objects of this invention,and the manner of attaining them, will become more apparent and theinvention itself will be better understood by reference to the followingdescription of embodiments of the invention taken in conjunction withthe accompanying drawings, wherein:

FIG. 1 is a block diagram illustrating an example of typical prior artRF receiver topology that results in the creation of inter-modulationartifacts.

FIG. 2 is a plot of the input power versus output power of a typicalnon-linear device.

FIG. 3 is an example expanded plot of output power versus input powerfor a typical non-linear device.

FIG. 4 is a plot of frequency versus a level of expected third orderinter-modulation.

FIG. 5 is a series of plots illustrating the third order intercept point(IP3) inter-modulation power increase versus frequency for non-lineardevices with no saturation.

FIG. 6 is a schematic illustration of IP3 inter-modulation powerincrease for non-linear devices with no saturation.

FIG. 7 is an amplitude versus frequency plot of inter-modulationartifacts in a radio frequency signal.

FIG. 8 is a block diagram of one embodiment of a single tuner radiosystem of the present invention.

FIG. 9 is a timing diagram depicting muting during a neighbor frequencycheck according to the present invention.

FIG. 10 is a table depicting one embodiment of a frequency learn memoryused to gather apriori information for the European market according tothe invention.

FIG. 11 is a table depicting one embodiment of a frequency learn memoryfor the North American market according to the invention.

FIG. 12 is a block diagram of one embodiment of a dual tuner radiosystem of the present invention.

FIG. 13 is a block diagram of one embodiment of a dual tuner phasediversity system of the present invention.

FIG. 14 is a block diagram of one embodiment of a dual tuner externalswitched diversity system of the present invention.

DETAILED DESCRIPTION

The embodiments hereinafter disclosed are not intended to be exhaustiveor limit the invention to the precise forms disclosed in the followingdescription. Rather the embodiments are chosen and described so thatothers skilled in the art may utilize its teachings.

In one embodiment, the method enables the radio to build up signal levelmetrics of frequencies in the FM band in a memory repository and thenutilize the information using the formulae such as (1) and (2) below inidentifying the artifact:

L*f1+/−M*f2+/−N*f3, where f1, f2 and f3 are distinct frequencies andL+M+N=3, where L, M and N are integers   (1)

OR from

L*f1+/−M*f2, where f1, f2 are distinct frequencies and L+M=3, where Land M are integers   (2)

The way the information is updated in the repository memory area mayvary between single and dual tuner.

Referring now to FIG. 8, there is shown one embodiment of a single tunerradio system 20 of the present invention. Radio system 20 may include amicrocontroller 22 which may be used to process user input. A digitalsignal processor (DSP) 24 may be used to provide audio demodulation ofthe air-borne intermediate frequency (IF) input signal. DSP 24 may alsobe used to provide quality information parameters to the mainmicrocontroller 22 via a serial communication protocol such as I2C. Thequality information parameters may include multipath, adjacent channelnoise, FM frequency offset, FM modulation and field strength. The I2Cchannel may be a dedicated channel so that delays due to shared resourcecontentions are prevented. DSP 24 may rely on a Tuner IF Front End IC 26to perform the front end RF demodulation and the gain control. Tuner IFFront End IC 26 may also output the Intermediate Frequency to DSP 24where the Intermediate Frequency may be demodulated and processed. TunerIF Front End IC 26 may further provide a gain to the IF (IntermediateFrequency) signal of up to 6 dBuV prior to forwarding the signal to DSP24. Communication between Tuner IF Front End IC 26 and DSP 24, asindicated at 27, may be via a serial communication protocol such as I2C,which may operate at 400 kbps.

An antenna system 28 may be communicatively coupled to Tuner IF FrontEnd IC 26. Antenna system 28 may be in the form of a passive mast, or anactive mast of phase diversity, for example.

An AF sample line 29 and an AF hold line 31 provide an interface betweenDSP 24 and Tuner IF Front End IC 26 to coordinate a quick mute asdescribed hereinbelow. A pause interrupt line 33 between DSP 24 andmicrocontroller 22 may be used to inform microcontroller 22 whenever apause occurs.

DSP 24 may provide signal quality parameterization of demodulated tuneraudio and may make it available to microcontroller 22 via a serialcommunication bus 30. In one embodiment, serial communication bus 30 isin the form of a 400 kbps high speed I2C.

The signal parameterization may include field strength, multipath, FMfrequency offset, FM modulation and ultrasonic noise. Field strength maygive an indication of signal reception and may help determine whetherthe radio station has good signal coverage in the vicinity of the user.This field strength quality parameter may be applicable for both AM andFM modulation signal reception.

Although the signal can have high field strength, it can be subject toreflections which can arise from trees and tall building whichreflect/deflect the signal. The multipath parameter may enable the levelof multipath to be ascertained, and may affect reception quality. Themultipath quality parameter may be more applicable to FM modulationsignal reception than to AM because in AM reception the wavelength iswider.

With regard to the ultrasonic noise quality parameter, it sometimeshappens that stations over-modulate their signal leading to adjacentchannel interference. For example, in the U.S., FM frequencies arespaced apart 200 kHz. There can arise times in which an adjacent stationover-modulates its signal past the 75 kHz modulation and beyond the 50kHz guard band, which may result in the adjacent station being heard onthe tuned-to station's frequency.

With regard to the FM modulation quality parameter, the detector mayprovide the amount of frequency deviation about the FM carrier centerfrequency. The amount of frequency deviation may be directlyproportional to the audio content being played in the FM station. Thetypical modulation bounds of this detect is 75 kHz for North America andbetween 22.5 kHz and 40 kHz for Rest of World and Europe. The FMmodulation quality parameter is discussed in more detail hereinbelow.

The quality parameter of FM frequency offset is a measure ofmisalignment between modulation and demodulation frequencies. Themisalignment value is typically small. However, a large offset error inthe form of a large misalignment value may signify strong adjacentchannel presence. Alternatively, a large offset error in the form of alarge misalignment value may signify that the transmitting station is a“pirate” station and is not operating exactly on its assigned frequency,but rather has an inherent offset error. This tends to occur in Italy.

A novel feature of the present invention is the sampling of FM signalswhile the user is listening to an FM signal as the current foregroundsource. The difficulty associated with performing the sensitivity checkwhile in FM mode, especially in a single tuner environment, is that thetuner to which the listener is listening has to momentarily switch toanother station, perform the quality check, and then re-tune to thelistened-to station. The user is not able to listen to the stationduring the time period between the switching of the station and there-tuning of the station. This interruption in the signal of thelistened-to station may be perceptible by the user, and thus may be asource of annoyance to the user.

If the audio system is in compact disc (CD) mode or is using some othernon-tuner source, the bandscan checks of the frequencies can be easilyperformed as the tuner can perform the checks without the checks beingperceptible to the user since the user is listening to a non-tunersource. To be able to perform the checks in an imperceptible manner, thepresent invention may utilize a DSP including pause detection logic thatis able to detect pauses (i.e., periods of silence or unvoiced activity)in the demodulated audio stream. In one embodiment, pause is detected bycomputing the number of zero crossings in a particular window of time,wherein a zero crossing may be defined as the value where the modulationdrops to zero or nearly zero. In addition, or alternatively, pause maybe detected by utilizing a signal strength threshold below which theaudio may be characterized as being in a pause. In one embodiment, apause may be recognized when the duration of the pause exceeds about 40milliseconds.

It may be assumed that the longer the period of time that a pause hasgone on, the longer the period of time that the pause will continue inthe future. Thus, a quality check may be initiated after a pause hasgone on for a predetermined period of time, such as 40 milliseconds, onthe assumption that the pause is more likely to continue long enough forthe quality check to be completed.

Each recognized pause may interrupt the main microprocessor, which maythen query a neighboring frequency for the quality value of theneighboring frequency. The quality value may be a function of multipath,signal strength, FM frequency offset, FM modulation and/or adjacentchannel noise (also termed “ultrasonic noise”).

FIG. 9 is a timing diagram depicting the muting during a neighborfrequency check triggered by the pause detection logic of DSP 24. Themuting may occur while the audio frequency (AF) Hold line is LOW, asindicated at 32. In the example illustrated in FIG. 9, the neighborfrequency check indicated at 32 has a duration of about 5.2 millisecondsusing Tuner IF Front End IC 26 interacting with DSP 24. The magnitude ofthe tuning voltage may be dependent on the neighbor frequency jump,i.e., on the frequency difference between the currently listened-tofrequency and the neighbor frequency to be checked. The overall timerequired to perform a neighbor check may be about seven milliseconds inone embodiment. The AF Hold line may go LOW in order to mute the audioprior to the actual tuning of Tuner IF Front End IC 26 to the particularneighboring frequency, which tuning is indicated at 34. After thecommencement of tuning, as indicated at 36, about one millisecond may beprovided for settling of phase-locked loop (PLL) locking prior to actualsampling being performed during the time that the AF Sample line goesHIGH, as indicated at 38. After the quality AF Sample check, the tuningfrequency may be set back to the originally listened-to station, asindicated at 40. After the tuning frequency is set back, time may beprovided for PLL setting before the AF Hold Line goes HIGH, as indicatedat 42, to unmute the audio of the presently listened-to station.

In one embodiment, after Tuner IF Front End IC 26 has switched to theneighboring frequency, as indicated at 34, the quality sample check isperformed to gather readings of the five parameters of fieldstrength,multipath, ultrasonic noise, FM frequency offset and FM modulation. Thereadings may be gathered via an I2C bus which is set at 400 kbps. Inorder to promote fast access and avoid having to make five consecutiveI2C reads from five separate and disparate memory locations in the DSPfor the fieldstrength, multipath, ultrasonic noise, FM frequency offsetand FM modulation parameters, DSP 24 may support calling the fiveregisters which hold this information through one I2C read. In order toenable the single I2C read, DSP 24 may support autoincrement and theability to map disparate memory locations via pointer access. Thesefeatures may be instrumental in performing the quality sample checkwithin the stipulated time frame and in avoiding the mute, i.e., theinterruption of the audible broadcast, from being perceived bypassengers of the vehicle.

When the quality sample check is performed on the neighboring frequency,the audio is muted for up to 5.2 milliseconds, i.e., the approximateduration of 32 in FIG. 9, which may be imperceptible by the user.

When the audio system is in tuner mode, each quality sample check maytake about seven milliseconds, which may be imperceptible to listenersso long as the quality sample checks are not performed consecutively,i.e., back to back, with no breaks in between. In one embodiment,precautions may be added in order to prevent or inhibit consecutivequality sample checks from being performed. Otherwise, consecutiveperformance of the checks could result in an interruption of the audiblebroadcast of greater than seven milliseconds, which could be perceptibleto the end user listeners.

Preventing checks from occurring consecutively (e.g., back to back) is afeature of the invention that may be applied to both automated FMstation list and AF switching methodology. In order to inhibit orprevent checks from being performed back to back or consecutively, whichcan result in the user perceiving the audio mute, a one-shot timer maybe set each time a check is performed. The setting of the one-shot timermay ensure that even if there were to be a pause detect triggerimmediately after a previous pause detect triggered check has beenperformed, the second check would be performed only if this timer haselapsed. Thus, the quality check may be an AND logic condition, meaningthat a pause has occurred AND the timer is not running. If pause occursand Timer is running, then the quality check is ignored. Thisconsecutive check prevention one-shot timer may be calibratable.

Ensuring quality check efficiency is another feature of the inventionthat may be applied to both automated FM station list and AF switchingmethodology. The FM frequency band in the North American market has 102frequencies ranging from 87.7 MHz to 107.9 MHz. In order to enhanceefficiency in the quality sample checks, a trust timer in software maybe utilized when quality check is performed on a station frequency toensure further checks are postponed in order to achieve checkefficiency. The timer value may be decremented using speed informationprovided by a vehicle local area network, or may be decremented byperiodic tick. As soon as a station has been sampled for quality, atimer associated with that particular station may be set. As long as thetimer is valid (i.e., has a non-zero value), a quality check may not beperformed again on that station. Once the timer decrements to zerohowever, another quality check may be performed.

The trust timer may be decremented either by periodic timer tick orthrough speed information provided by the local area network within thecar. The timer decrement via speed information may be particularlyadvantageous in one embodiment because if the vehicle is stationarythere is no decrement of the timer. The rate of decrement may bedependent upon the speed of the vehicle.

For example, it is possible to sample station 87.5 MHz 0 (index of 87.5MHz) and an associated trust timer for about fifteen, which time iscalibratable. Subsequent checks ignore checking 87.5 MHz until its trusttimer expires.

A table depicting one embodiment of a frequency learn memory used togather apriori information is shown in FIG. 10. The learn memory is therepository from which the subsequent logic may be derived. The learnmemory may include 102 entries for the U.S. region (e.g., 87.7 MHz to107.9 MHz with 200 kHz steps), 205 entries for the worst case FM range(e.g., 87.5 MHz to 108 MHz with 100 kHz steps), and 140 entries for theJapan region (e.g., 76 MHz to 90 MHz with 100 kHz steps).

The invention may be applied to perceptually weighted checks. Tocomplement the pause detect logic check, the invention provides amethodology which triggers a neighborhood frequency check when thecurrently listened-to station has poor reception quality. Moreparticularly, when the currently listened-to station has poor receptionquality, the present invention may “sneak in” a performance check thatis not easily perceived by the user. In order to enable such checks, aperceptual weighting filter based on the quality parameter is utilized.The perceptually weighted checks take advantage of the poor signalreception of the presently listened-to station to perform checks.

In order to support the checks, a one shot timer having a duration of500 ms is used to continuously check on the current quality state of thecurrently tuned-to station in FM mode. If the quality state indicatesnoise AND a previous quality check was not performed within the onesecond time frame, then a quality check is initiated. This one secondcheck guard may ensure that back to back quality checks are notperformed, because such back to back checks could be perceived by theuser.

The perceptual filter that may be utilized includes a three-dimensionalfunction which inputs field strength, multipath and ultrasonic noiseinto a quality factor. The three parameters may be received from the DSPthrough autoincrement registers.

The quality information gathered may be updated into what may be termeda “frequency learn memory,” which is mapped onto on-chip RAM. Oneembodiment of a frequency learn memory for the North American market isshown in FIG. 11.

To optimize on RAM, instead of storing frequency, each frequency may bepresented as an index that is mapped over the range. For example, in afrequency range spanning from 87.7 MHz to 107.9 MHz, index 0 representsfrequency 87.7 MHz, and index 102 represents 107.9 MHz. To otherwisestore the frequency uncoded in BCD format, for example, would consumetwo bytes, which is not an efficient use of memory.

Quality may be derived from the three-dimensional table taking intoconsideration fieldstrength, multipath and ultrasonic noise. The trusttimer may be a timer value that gets set once a quality check has beenperformed on a station.

The learn memory may be updated through the following four methods on asingle tuner radio. First, when a user is tuned to an FM station and thevolume knob is set to a perceivable volume level, then automatic qualitychecks of neighboring frequencies may be triggered whenever there is apause in the currently tuned-to station's audio. The novelty of thisidea is extended in the second through fourth options described below.

A second option for the automatic update of the FM station list is thatwhen a user is tuned to an FM station and the volume knob is set to aperceivable volume level, then automatic quality checks of neighboringfrequencies may be triggered whenever the currently tuned-to audiosignal quality is poor. In one embodiment, the present inventionprovides a novel perceptual based table which characterizes the signalquality level. The characterization of the signal quality level may beused to trigger a 7 ms long, unperceivable quality check of aneighboring frequency.

A third option for the automatic update of the FM station list is thatwhen a user is tuned to an FM station and the volume knob is set tototal mute (or if a mute pushbutton is activated), then the neighboringfrequencies are checked and updated onto the FM learn memory.

A fourth option for the automatic update of the FM station list is thatwhen a user is sourced to a non-tuner source (e.g., CD mode, auxiliarymode), then the update of the FM station list can freely be performedwithout the concern that the update will be perceived by the user. Dualtuner radios may not have this limitation, as the second tuner can scanthe FM memory and keep it updated.

The invention may be applied to AF switching methodology in a dual tunerradio. A dual tuner radio system 120 of the present invention isillustrated in FIG. 12. Dual tuner radio system 120 may include amicrocontroller 122 which may be used to process user input. A digitalsignal processor (DSP) 124 may be used to provide audio demodulation ofthe air-borne IF input signal. DSP 124 may also be used to providequality information parameters to the main microcontroller 122 via aserial communication protocol such as I2C. The quality informationparameters may include multipath, adjacent channel noise, FM frequencyoffset, FM modulation and field strength. The I2C channel may be adedicated channel so that delays due to shared resource contentions areprevented. DSP 124 may rely on a Two-tuner IC 126 to perform the frontend RF demodulation and the gain control. Two-tuner IC 126 may alsooutput the Intermediate Frequency to DSP 124 where the IntermediateFrequency may be demodulated and processed. Two-tuner IC 126 may furtherprovide a gain to the IF (Intermediate Frequency) signal of up to 6 dBuVprior to forwarding the signal to DSP 124. Communication betweenTwo-tuner IC 126 and DSP 124, as indicated at 127, may be via a serialcommunication protocol such as I2C, which may operate at 400 kbps.

An antenna system 128 may be communicatively coupled to Two Tuner IC126. Antenna system 128 may be in the form of a passive mast, or anactive mast of phase diversity, for example.

AF sample lines 129 a-b and AF hold lines 131 a-b provide an interfacebetween DSP 124 and Tuner IC 126 to coordinate a quick mute as describedhereinbelow. In contrast to the single tuner embodiment of FIG. 8, thisdual tuner embodiment of FIG. 12 includes a separate AF Sample, AF Holdand Pause sensor for the second tuner path. Pause interrupt lines 133a-b between DSP 124 and microcontroller 122 may be used to informmicrocontroller 122 whenever a pause occurs either on the primary orsecondary tuner paths.

DSP 124 may provide signal quality parameterization of demodulated tuneraudio and may make it available to microcontroller 122 via a serialcommunication bus 130. In one embodiment, serial communication bus 130is in the form of a 400 kbps high speed I2C.

For dual tuner variants, second tuner may be used to conduct the PIcheck in an unperceived manner since the user is listening to the maintuner for the audio source. This allows the frequency learn memory to beupdated with respect to quality metrics more easily than with singletuner radios, especially when the user is sourced to either AM or FMsource.

Dual tuner radio variants can be of either the phase diversity type orthe external switching diversity type. On dual tuner variants with phasediversity (FIG. 13), a main tuner 226 is connected to an antenna 228a,and a second tuner 227 is connected to an antenna 228b. While main tuner226 produces an audio signal, second tuner 227 can scan the FM spectrumin the background until the main tuned-to station experiences severemultipath. In response to the severe multipath, the background scanningmay be ceased and second tuner 227 may tune to the same station thatmain tuner 226 is tuned to. Thus, the audio quality may be enhanced byusing algorithms known as Constant Modulus Algorithm (CMA) that make useof the phase differences between the main tuner demodulated audio andthe second tuner demodulated audio. For dual tuner variants with phasediversity, whenever the phase diversity is functionally enabled, thedual tuner in part operates mostly as a single tuner radio.

On dual tuner variants with external switching diversity (FIG. 14), amain tuner 326 and a second tuner 327 are associated with antennas 328a-b. While main tuner 326 produces an audio signal, second tuner 327 isconstantly engaged in background scanning. The diversity in tunervariants with external switching diversity is a front end switchingcircuitry box 334 which chooses the better antenna signal quality. Forexample, as shown in FIG. 9, box 334 determines that antenna 328 a isthe stronger antenna, and thus chooses antenna 328 a, as indicated at336.

The frequency learn memory contains the updated information of thestation frequency landscape that is currently available to the carradio. The invention provides different methods of updating the learnmemory by use of single and dual tuners.

Using the quality metrics gathered in the frequency learn memory, theinventive system can employ various methods to detect the existence ofan inter-modulation artifact. A first method of detecting aninter-modulation artifact includes inter-modulation detection, in whichthe learn memory may be checked through for all frequencies above acalibratable threshold, such as 70 dBuV for example.

In a second method of detecting an inter-modulation artifact, if thefrequency signal quality is greater than or equal to 70 dbuV, and if thenumber of stations found equals two, then third order 2f1+/−f2 and2f1+/−f2 combinations are computed. It may be checked whether thefrequency is within range of the FM band, which varies based on theregion. The FM band is 87.5 to 108.0 MHz for Europe (ECE) and rest ofworld (ROW); 76 to 90 MHz for Japan; and 87.75 to 107.9 MHz for theNorth American market.

In a third method of detecting an inter-modulation artifact, if thenumber of stations found equals three, then combinations of f1+/−f2+/−f3are computed and a check is made that the frequencies are within rangeof the respective tuner region (e.g., 87.7 to 107.9 MHz in the U.S.; 76to 90 MHz in Japan; and 87.5 to 108.0 MHz in the Rest Of World). If thefrequencies are within range of the respective tuner region, then a bitis set for these frequencies in learn memory along with a trust timer.For example, a valid count down timer may be set for fifteen minutes, orsome other chosen time period. As long as the trust timer is running,the radio may be able to judge this station and skip this stationfrequency in Autoseek, AF switching and DAB FM link use cases.

The present invention may improve the tuner reception qualityperformance by avoiding third order inter-modulation artifacts in singleand dual tuner radio variants in the presence of strong signalenvironment. The inventive method can be applied to car radios, and FMreceivers in mobile devices such as cell phones, USB—FM receivers, etc.

The inventive method for detection of inter-modulation uses aprioriinformation in improving several different applications. First, RDS AFswitching behavior may be improved in single and dual tuner radios byensuring that the radio does not switch to a tainted inter-modulationfrequency.

Second, RDS preset recall performance may be improved by using the Tuneby PI code to ensure that the alternative frequency picked for receptionis not a frequency tainted by inter-modulation artifacts.

Third, Auto-seek seek stop performance may be improved in the FM mode toensure that seek stop does not occur at an inter-modulation artifact.

Fourth, in Europe, DAB FM link occurs when a user is tuned to a digitalDAB station. When the BER (Bit Error Rate) increases, the decoding ofthe MP2 compressed audio stream becomes difficult for the DAB receiver.In such a circumstance, the radio typically falls back on the simulcastFM station frequency to produce audio. FM stations in Europe employ RDSwhich categorizes stations with a program ID code whereby multiplefrequencies are associated with a single station. In such a case, a Tuneby PI operation to trigger the DAB FM link may ensure that the finalstrongest alternative frequency picked for tune operation in the FM bandis not an inter-modulation artifact.

Fifth, the invention may reduce effects of inter-modulation in thescenario where the user manually tunes to a station, and the radiocomputes the station to be a known inter-modulation tainted stationfrequency. For example, the radio may narrow the bandwidth of filteringin order to filter out the inter-modulation artifact. If the radiodetermines that it is tuned to a frequency that is itself aninter-modulation artifact, then the radio may switch to one of the“pure” frequencies that contribute to the inter-modulation artifact.

While this invention has been described as having an exemplary design,the present invention may be further modified within the spirit andscope of this disclosure. This application is therefore intended tocover any variations, uses, or adaptations of the invention using itsgeneral principles. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this invention pertains.

What is claimed is:
 1. A radio comprising: a tuner configured to tune toa primary frequency; and a controller configured to: determine whether acandidate alternate frequency is a third order inter-modulationartifact; and cause the tuner to switch tuning from the primaryfrequency to the candidate alternate frequency only if it is determinedthat the candidate alternate frequency is not a third orderinter-modulation artifact.
 2. The radio of claim 1 wherein thecontroller is configured to ascertain whether content of the candidatealternate frequency is equivalent to content of the primary frequency,the ascertaining being performed only if it is determined that thecandidate alternate frequency is not a third order inter-modulationartifact, the controller being configured to cause the tuner to switchtuning from the primary frequency to the candidate alternate frequencyonly if it is ascertained that the content of the candidate alternatefrequency is equivalent to the content of the primary frequency.
 3. Theradio of claim 2 wherein the ascertaining and the switching areperformed during a pause in a currently tuned-to frequency.
 4. The radioof claim 2 wherein the ascertaining is performed only if it isdetermined that a signal quality of the candidate alternate frequencyexceeds a threshold signal quality level.
 5. The radio of claim 1wherein the controller is configured to perform a program identificationcheck for the candidate alternate frequency only if it is determinedthat the candidate alternate frequency is not a third orderinter-modulation artifact.
 6. The radio of claim 1 wherein thecontroller is configured to identify two frequencies f1 and f2 within anFM band that have a signal quality above a threshold level, thedetermining including determining whether the candidate alternatefrequency equals either:L*f1+M*f2; orL*f1−M*f2 wherein L and M are integers and L+M=3.
 7. The radio of claim1 wherein the controller is configured to identify three frequencies f1,f2 and f3 within an FM band that have a signal quality above a thresholdlevel, the determining including determining whether the candidatealternate frequency equals either:L*f1+M*f2+N*f3;L*f1+M*f2−N*f3;L*f1−M*f2+N*f3; orL*f1−M*f2−N*f3 wherein L, M and N are integers and L+M+N=3.
 8. The radioof claim 1 wherein the controller is configured to cause the tuner totune to the candidate alternate frequency in an autoseek operation onlyif it is determined that the candidate frequency is not a third orderinter-modulation artifact.
 9. The radio of claim 1 wherein thecontroller is configured to cause the tuner to switch tuning from theprimary frequency to the candidate alternate frequency only if it isascertained that a signal quality of the candidate alternate frequencyis better than a signal quality of the primary frequency.
 10. A radiocomprising: a tuner configured to scan a radio frequency band for acandidate frequency having a quality exceeding a threshold qualitylevel; and a controller coupled to the tuner and configured to:determine whether the candidate frequency is a third orderinter-modulation artifact; and cause the tuner to tune to the candidatefrequency only if it is determined that the candidate frequency is not athird order inter-modulation artifact.
 11. The radio of claim 10 whereinthe controller is configured to cause the tuner to switch tuning from aprimary frequency to the candidate frequency in an alternate frequencyswitching operation only if it is determined that the candidatefrequency is not a third order inter-modulation artifact.
 12. The radioof claim 10 wherein the scanning includes scanning the radio frequencyband for a candidate frequency having a signal quality metric exceedinga threshold signal quality metric, the signal quality metric beingdependent upon at least one of field strength, multipath, adjacentchannel energy, frequency offset and FM modulation.
 13. The radio ofclaim 10 wherein the controller is configured to identify twofrequencies f1 and f2 within an FM band that have a signal quality abovea threshold level, the determining including determining whether thecandidate frequency equals either: L*f1+M*f2; orL*f1−M*f2 wherein L and M are integers and L+M=3.
 14. The radio of claim10 wherein the controller is configured to identify three frequenciesf1, f2 and f3 within an FM band that have a signal quality above athreshold level, the determining including determining whether thecandidate frequency equals either:L*f1+M*f2+N*f3;L*f1+M*f2−N*f3;L*f1−M*f2+N*f3; orL*f1−M*f2−N*f3 wherein L, M and N are integers and L+M+N=3.
 15. A radiocomprising: a tuner; and a controller configured to: identify a firstfrequency within an FM band that has a signal quality above a thresholdlevel; calculate a second frequency that is a third orderinter-modulation artifact of the first frequency; and inhibit the tunerfrom tuning to the second frequency.
 16. The radio of claim 15 whereinthe inhibiting is performed in an alternative frequency switchingoperation.
 17. The radio of claim 15 wherein the inhibiting is performedin an autoseek operation.
 18. The radio of claim 15 wherein thecontroller is configured to determine the signal quality of the firstfrequency by measuring field strength, multipath, adjacent channelenergy, frequency offset and/or FM modulation of the first frequency.19. The radio of claim 15 wherein the controller is configured toidentify a plurality of first frequencies within an FM band that have asignal quality above a threshold level, the calculating includescalculating, for two said first frequencies f1 and f2:L*f1+M*f2; andL*f1−M*f2 for each combination of L and M wherein L and M are integersand L+M=3.
 20. The radio of claim 15 wherein the controller isconfigured to identify a plurality of first frequencies within an FMband that have a signal quality above a threshold level, the calculatingincludes calculating, for three said first frequencies f1, f2 and f3:L*f1+M*f2+N*f3;L*f1+M*f2−N*f3;L*f1−M*f2+N*f3; andL*f1−M*f2−N*f3 for each combination of L, M and N wherein L, M and N areintegers and L+M+N=3.